Glass compositions having various body colors are used, for example, by architects in glazing buildings. Normally, the color selected by the architect serves several functions. A first function of the color is to make the glass aesthetically pleasing when viewed from the exterior of a building. Aesthetics will determine the acceptability of a desired particular glass color and, in part, the desired intensity of the color. A second function is to reduce the amount of heat absorbed from the exterior of the building to the interior of the building, so that the air conditioning load in the building is reduced. Generally, more color added to glass results in greater heat absorption. In addition, while color may readily be added to glass to serve these functions, the glass when colored still must have an appropriate visible light transmission value. Also, the glass must be structurally sound.
Those skilled in the art of formulating glass compositions are familiar with various suitable glass colorants. Thus, a small amount of cobalt oxide is known to produce a cold blue color widely considered unattractive and undesirable for architectural and certain other uses. A high concentration of nickel is known to produce black glass. Selenium can produce pink or red and ferrous oxide can produce green glass and contribute to advantageous solar load control properties. The glass color produced by a mixture of colorants will vary with both the amount and relative proportions used. As noted above, varying the choice and amount of colorants also affects the light transmission value of the glass. As discussed further below, however, the effect on glass color and transmission value of mixing multiple colorants and/or varying the amount or relative proportions of the colorants often is not reliably predictable.
In the following discussion, certain terms well known to the skilled of the art are used to describe color in glass. Two terms or specifications for color, dominant wavelength and excitation purity, are derived from tristimulus values that have been adopted by the International Commission on Illumination. The numerical values of these two specifications for a given glass color can be determined by calculating the trichromatic coefficients, X, Y and Z, from the so-called tristimulus values of that glass color. The trichromatic coefficients X and Y then are plotted on a chromaticity diagram and numerically compared with the coordinates of Illuminant C, an established standard light source. (The trichromatic coefficient Z value can be obtained by adding X and Y and subtracting the total from 1.0.) This comparison provides the numerical values of the excitation purity and dominant wavelength of the glass color.
Thus, a glass color may be specified either by its coefficients X and Y or by its dominant wavelength and purity values. The lower the excitation purity of a color, the closer it is to the Illuminant C standard and the closer it is to being a so called neutral color.
An understanding of the foregoing terms and definitions thereof may be had by referring to the Handbook of Colorimetry prepared by the staff of the Color Measurement Laboratory, Massachusetts Institute of Technology. This book was printed in 1936 by the Technology Press, Massachusetts Institute of Technology, Cambridge, Mass. Also, a good explanation and list of definitions is given in Color in Business, Science and Industry, (3 Ed.) John Wiley & Sons (especially pages 170-72, 377-78). Useful also is An Introduction to Color, John Wiley & Sons (especially pages 105-106).
Those skilled in the art know that adding or substituting one colorant for another and/or changing the amount or relative proportion of colorants in a glass composition affects not only the color of the glass, i.e., its dominant wavelength and its excitation purity, but also can affect the light transmission (T) of the glass and its structural qualities. Furthermore, there is in many cases substantial complexity and unpredictability in these effects. Thus, for example, even if the proper alteration in the composition of a particular colored glass were determined for achieving a desired color shift, the same alteration, unfortunately, would also alter (for example, unacceptably reduce) the light transmission value of the glass. It may, of course, also undesirably alter the purity of the glass color. In short, all these factors--dominant wavelength, purity and light transmission--are variable and may vary unpredictably with each other. Developing a new glass composition, therefore, having a particular color, certain purity and appropriate light transmission value, is in some cases like searching for a needle in a haystack. An experimental change in the amount or relative proportions of one or more colorants in a glass composition intended to bring one of these numerical values closer to a target value causes one or both the other values simultaneously to drift off target (or further off target).
The difficulty of this task, finding the correct glass composition for a body colored glass having the desired dominant wavelength and excitation purity value and light transmission value, is discussed in U.S. Pat. No. 3,296,004 to Duncan, wherein a neutral brown heat absorbing glass is disclosed. Duncan expressly notes that the development of the particular color required a careful consideration of the transmittance characteristics of the glass and that the amounts of the colorants must be carefully controlled to achieved the desired color (dominant wavelength and excitation purity), transmittance and heat-absorbing characteristics. Thus, for example, Duncan points out that if his glass contained more cobalt oxide than he specifies, the color would be more blue than desired. Considering the glass composition of the present invention for a moment, however, the great unpredictability of this area is well demonstrated by the fact that it employs cobalt oxide in an amount well within the range used by Duncan, yet achieves a blue, not a brown color. That is, the amount of cobalt oxide used by Duncan to produce brown surprisingly yields the attractive neutral blue color of the present invention in combination with the other components of the glass composition of the present invention, notwithstanding that such other components are not normally associated with producing blue coloration.
This inherent unpredictability in achieving specific purity, dominant wavelength and light transmission values simultaneously in a structurally sound glass composition had to be overcome in discovering the glass composition of the present invention. A series of blue glass compositions were fabricated for aesthetic evaluation for automotive and architectural uses and the like. They were exhibited to potential users including numerous architectural firms. From amongst the many samples, one was chosen having a very attractive neutral blue color--the color of the glass composition of the present invention. In particular, it was a blue having a dominant wavelength of 482 nm.+-.1 nm and a purity of 13%.+-.1%. These values correspond to the color coordinates of the glass, i.e., trichromatic coefficients X=0.2799 and Y=0.2947.
At this point, the search for the needle in the haystack, the glass composition of the present invention, had really just begun. The light transmissability of the exhibited glass samples had been largely ignored for purposes of the exhibit so that a desired glass coloration could first be identified. The light transmission value of the selected neutral blue color sample was unacceptable for many intended commercial applications. The task now was to develop a glass composition including appropriate colorants which yielded the same dominant wavelength and the same excitation purity, but with the needed visible light transmission value. As explained above, however, altering the various colorants and the amount and relative proportions in which they were used for purposes of achieving the correct light transmission value simultaneously caused the color, that is the dominant wavelength and purity, to drift off target.